Steam generator



June 8, 1965 sT ETAL 3,187,725

STEAM GENERATOR Filed Sept. 10, 1963 19. 1 16 ZOJ INVENTORS ATTORNEY United States Patent 3,187,725 STEAM GENERATOR Erich Stange, Hamburg-Poppenhuttel, and Heinrich Dorfmann, Ratingeu, Germwy, assignors to Burrwerhe Ahtiengesellschaft, Ratingeu, Germany, a corporation of Germany Filed Sept. 10, 1963, Ser. No. 307,913 Claims priority, application Germany, Sept. 10, 1962, D 39,817 5 Claims. (Cl. 122-4) This invention relates to steam generators and more particularly to a system for superheating steam in a high pressure high temperature steam generator, while inhibiting external corrosion of the steam superheating surfaces.

The use of high pressure steam for power generation involves superheat temperatures up to the practical limit in order to increase the available heat. Practical limits of steam temperature and pressure are primarily governed by the materials used in superheater construction. During the past few years there has been considerable development in the metallurgy of steel and alloys, to the extend that it is now possible to use steam of the order of approximately 1120" F. in power generation.

The economy of the use of high steam pressures and temperatures is further enchanced by the generation and expansion of the steam in large central steam plants. Pulverized coal-fired furnaces, especially those designed for removal of ash in molten form, and oil-fired furnaces are generally considered the most suitable for steam generators of large output. Natural gas-fired furnaces are also appropriate for high capacity installations if there is an assured supply of economical natural gas. Stokerfired furnaces, however, are excluded for such applications because of their comparatively limited capacity; and pulverized coal-fired furnaces designed for dry ash removal are at the limit of their usefulness.

With the advent of higher steam temperatures, superheater tube metal temperatures have reached the point where tube metal wastage has occurred in certain units firing oil and/or pulverized coal in a furnace designed for liquid removal of coal ash. In operation of these units, ash particles combined either in solid liquid or gaseous form with the materials of the superheater tubes or diffused into them, thereby destroying the properties of the tube materials which make them especially useful. In these case of pulverized coal furnace designed for liquid removal of coal ash, the sulfuric compounds formed with calcium and silicon as volatile or liquid sulfides cause the destruction; while in the case of oil-tired furnaces, the steels are mainly attacked by the vanadium content of the ash, particularly when the sulfur content is high. These phenomena are generally characterized as high temperature corrosion for corrosion is a function of tube metal temperature, increasing with increasing temperature. The use of superheated tube materials neutral against attack or" this would substantially increase the cost of the superheater, even though the heating surface requiring such neutral tube materials would amount to a fraction of the total heating surface of the steam generator since corrosion only starts above a tube metal temperature of approximately 1004 F. However, the heating surface subject to corrosion attack is extended by the repeated high reheat of the steam in several stages, such practice being considered a necessity in high pressure power generation.

The problem of high temperature corrosion is solved, in accordance with the present invention by providing a steam generator wherein a relatively low steam temperature superheater and a relatively high steam temperature superheater are respectively disposed in divided and separately fired parallel gas flow paths, with the low steam 3,187,725 Patented June 8, 1965 temperature being connected for series flow of steam to the high temperature superheater. Provisions are made for firing the gas flow path occupied by the low steam temperature superheater with a fuel having corrosive constituents and passing the resulting combustion gases in indirect heat transfer relation with the steam flowing through the flow steam temperature superheater, with the low steam tel iperature superheater being proportioned and arranged so that its tube metal temperature does not exceed that at which the corrosive constituents of the fuel would attack the tube metal of the low steam temperature superheater. The gas flow path occupied by the high steam temperature superheater is fired with a fuel substantially free of corrosive constituents so that the resulting combustion gases are of a character which do not attack tube materials at high tube wall temperatures, with the combustion gases so generated being passed in indirect heat transfer relation with the high steam temperature superheater, and with the high steam temperature superheater being proportioned and arranged so that the steam received from the 10v. steam temperature superheater is additionally superheated to the extent that the corresponding tube metal temperature exceedsvthat at which corrosion attack of the metal of the high temperature superheater would occur if the fuel used to generate the stream of combustion gases passing over the high steam temperature superneater were of a corrosive character.

This solution is also suited to control the final steam temperature of the superheaters by controlling the firing rate in the respective parallel flow gas flow paths. For economic reasons the final steam temperature should not be decreased at partial loads; nor should they be. exceeded for operational safety reasons. While it is possible to remove the corrosive constituents of a particular fuel by means of chemical or physical processes, this practice should be restricted to the fuel supply for the high temperature stage of the superheater since it entails a considerable increase in fuel expense.

Although it is Well known in the steam generator art to provide separately fired parallel flow gas flow paths respectively occupied by superheater surface, such arrangement is used merely to control the steam temperatures of the superheaters by regulating firing rates or by altering positions of burners or to burn two different kinds of fuel without regard to their corrosive or noncorrosive character and without regard to corrosion limits of the superheater SUTJIZICES. Thus the basic the invention was neither disclosed or solved.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawing and descriptive matter in which there is illustrated and described a preferred embodiment of the invention.

The accompanying drawing is a schematic representation of fluid heating circuits constructed and operable in accordance with the invention and particularly adapted for use in a forced circulation once-through steam generator. According to the invention, the fluid heating surfaces are divided into two separate parallel flow gas flow paths it) and 12. Gas flow path 10 is provided with a furnace l4 fired by a burner 16, while gas flow path 12" is provided with a furnace l8 fired by a burner 24). 'The steam generator is of the once-through forced-flow type having a series fluid flow path including a conduit 22 supplying feed water from any suitable source in parallel flow relation to a pair of fluid heating circuits 26 and 28 respectively occupying gas flow paths 1! and 12, with circuit 26 comprising an economizer 30 and a vapor generating section 32 and circuit 28 including an economizer ice problem of 34 and vapor generating section 36. If desired, all of the economizer and vapor generating surfaces can be disposed in gas flow path 12. Steam generated in the parallel flow circuits 26 and 28 passes to and through the low steam temperature stage 36 of a superheater disposed in gas flow path 12, then passes to and through a high steam temperature stage 40 of the superheater situated in gas flow path 10, and then flows to the high pressure stage of a steam turbine, not shown. Partially expanded steam from the turbine successively passes through low and high steam temperature stages 42 and -34 ofa reheater and then returns to the turbine for final expansion, with stage 42 being disposed in gas flow path 12 and stage 44 being situated in gas flow path 10.

The furnace of gas flow path 10 is especially adapted for the firing of a fuel producing combustion gases substantially free of non-corrosive constituents. Corrosive combustion gases are considered as those which attack the superheater tube materials when the metal temperatures are above approximately 1004 F. Thus the furnace 14 can be constructedand arranged to fire ash-free oil, .natural gas, pulverized-coal yielding solid ashes, or solid fuel with a stoker. Furnace 18 of gas flow path 12 is particularly adapted for burning commercial heating oil with vanadium content or pulverized or crushed coal under conditions where the ash is removed in molten form. The combustion gases produced in furnace 13 would attack the tubes of a superheater if their metal temperatures exceeded 1004 F. However, these gases are harmless for the superheating surfaces arranged in gas flow path 12 because the metal temperatures of the tubes constituting the surfaces at all times remain below the dangerous corrosion limit of 1004 F.

The connection in parallel of the fluid heating sections 39 and 34 as wellas 32 and 36 in the flow of the working medium is not a necessity. It may be even advantageous to connect them in series in order to prevent'uneven flow distribution in them during start-ups when only furnace 18 is in operation. Each of the heating surface sections represented as simple tube lines actually consists of a number of tubes connected in parallel. They are interconnected directly or by means of headers, manifolds and connecting tubes.

The separate furnaces 14 and 18 also permit control of the final steam temperature by adjustment of the firing rate. In addition, in the case of a forced circulation steam generator the finalsteam temperature may be controlled independently of the way of firing by changing the amount of feed water supply to the generator.

The arrangement of the fluid heating surface does not represent a characteristic sequence of heating gas flow in the gas flow paths. The use of vapor generating surface, and possibly economizer and superheater surfaces, for cooling the walls of the furnaces will be necessary in many cases if they consist of a combustion chamber, a secondary combustion chamber and a radiation chamber. The invention can be used for forced circulation steam generators as well as for steam generators with natural circulation. In certain gases the high temperature stages of several steam generators can be arranged in gas flow path 10. The combustion gases discharging from gas flow paths 10 and 12 may be merged in a common flue after passing over the heating surfaces particularly susceptible to corrosion. Also, the solid ash produced and accumulated in furnace 14 and gas flow path 10 may be introduced into furnace 18 in order to melt it.

While in accordance with the provisions of the statutes there is illustrated and described herein the best form and mode of the'operation of the invention now known, those skilled in the art'will understand that changes may be made in the form of the apparatus disclosed without departing from the spirit of the invention covered by the claims and that certain features of the invention may sometimes be used to advantage without a corresponding use of other features.

We claim: 1. The method of superheating steam in a steam generator having a relatively low steam temperature tubular superheater and a relatively high steam temperature tubular superheater disposed respectively in divided and separate parallel gas flow paths, which comprises passing steam through the low steam temperature superheater while passing a first stream of combustion gases generated by the firing of a fuel having corrosive constituents in indirect heat transfer relation with the steam flowing thnough the low steam temperature superheater, superheating the steam passing through the low steam temperature superheater to the extent that the corresponding tube metal temperature does not exceed that at which the corrosive constituents of said first stream of combustion gases attack the metal of said low steam temperature superheater, passing the steam discharging from the low steam temperature superheater to and through the high temperature superheater while passing a second stream of combustion gases generated by the firing of a fuel sub stantially free of corrosive constituents in indirect heat transfer relation with the steam flowing through the high steam temperature superheater, said first and second streams of combustion gases being completely separate from and independent of each other while passing over their corresponding superheaters, and additionally superheating the steam passing through the high steam temperature superheater to the extent that the corresponding tube metal temperature exceeds that at which corrosion attack of the metal of the high steam temperature superheater would occur if the fuel used to generate the second stream of combustion gases were of a corrosive character. 2. The method of claim 1 wherein the first stream of combustion gases are generated by the firing of a vanadium-containing oil.

3.,The method of claim 1 wherein the first stream of combustion gases are generated by the. firing of an ashcontaining solid fuel in a furnace under a mean furnace temperature above the fuel ash fusion temperature.

4. The method of claim 1 wherein the first stream of combustion gases are generated by the firing of an ashcontaining fuel in a furnace proportioned and arranged for removal of fuel ash in molten condition, and the second stream of combustion gases aregenerated by the firing of a fuel substantially free of ash.

5. In a steam generator, means forming a pair of separately fired parallel flow gas flow paths, a relatively low steam temperature tubular superheater disposed in one of said gas flow paths, a relatively high steam temperature superheater disposed in the other of said gas flow paths and connected for series flow of fluid from said low steam temperature superheater, means for generating and passing steam to said low steam temperature superheater, means for firing said one gas flow path with a fuel having corrosive constituents and causing the resulting combustion gases to pass in indirect heat transfer relation with the steam flowing through the low steam temperature superheater, said low steam temperature superheater being so proportioned and arranged that the steam flowing therethrough is superheated to'the extent that the corresponding tube metal temperature does not exceed that at which the corrosive constituents of said combustion gases attack the metal of said low steam temperature superheater, and means for firing said other gas flow path with a fuel substantially free of corrosive constituents and causing the 7 resulting combustion gases to pass in indirect heat transfer relation with the steam flowing through the high steam temperature superheater and completely separate from the combustion gases flowing over the low steam temperature superheater, said high steam temperature superheater being so proportioned and arranged that the steam flowing therethrough is additionally superheated to the extent that the corresponding tube metal temperature exceeds that at which corrosion attack of the metal of the high steam temperature superheater would occur if the fuel used to fire said other gas flow path were of a corrosive character.

References Cited by the Examiner UNITED STATES PATENTS 2,614,541 10/52 Armacost et a1 122-240 3,017,870 1/62 Profos 122-479 3,048,154 8/62 Braddy 1227 6 OTHER REFERENCES Evans, C. T. In: Oil Ash Corrosion of Materials at Fifty-Third Annual Meeting of the American Society for Testing Materials, June 26, 1950. Reprinted in special Technical Bulletin 108, published by the American Society for Testing Materials, pp. 59-105, page 63 relied on.

ROBERT A. OLEARY, Primary Examiner. MEYER PERLIN, Examiner. 

1. THE METHOD OF SUPERHEATING STEAM IN A STREAM GENERATOR HAVING A RELATIVELY LOW STEAM TEMPERATURE TUBULAR SUPERHEATER AND A RELATIVELY HIGH STEAM TEMPERATURE TUBULAR SUPERHEATER DISPOSED RESPECTIVELY IN DIVIDED AND SEPARATE PARALLEL GAS FLOW PATHS, WHICH COMPRISES PASSING STEAM THROUGH THE LOW STEAM TEMPERATURE SUPERHEATER WHILE PASSING A FIRST STREAM OF COMBUSTION GASES GENERATED BY THE FIRING OF A FUEL HAVING CORROSIVE CONSTITUENTS IN INDIRECT HEAT TRANSFER RELATION WITH THE STEAM FLOWING THROUGH THE LOW STEAM TEMPERATURE SUPERHEATER, SUPERHEATING THE STEAM PASSING THROUGH THE LOW STEAM TEMPERATURE SUPERHEATER TO THE EXTENT THAT THE CORRESPONDING TUBE METAL TEMPERATURE DOES NOT EXCEED THAT AT WHICH THE CORROSIVE CONSTITUENTS OF SAID FIRST STREAM OF COMBUSTION GASES ATTACK THE METAL OF SAID LOW STEAM TEMPERATURE SUPERHEATER, PASSING THE STEAM DISCHARGING FROM THE LOW STEAM TEMPERATURE SUPERHEATER TO AND THROUGH THE HIGH TEMPERATURE SUPERHEATER WHILE PASSING A SECOND STREAM OF COMBUSTION GASES GENERATED BY THE FIRING OF A FUEL SUBSTANTIALLY FREE OF CORROSIVE CONSTITUENTS IN INDIRECT HEAT TRANSFER RELATION WITH THE STEAM FLOWING THROUGH THE HIGH STEAM TEMPERATURE SUPERHEATER, SAID FIRST AND SECOND STREAMS OF COMBUSTION GASES BEING COMPLETELY SEPARATE FROM AND INDEPENDENT OF EACH OTHER WHILE PASSING OVER THEIR CORRESPONDING SUPERHEATERS, AND ADDITIONALLY SUPERHEATING THE STEAM PASSING THROUGH THE HIGH STEAM TEMPERATURE SUPERHEATER TO THE EXTENT THAT THE CORRESPONDING TUBE METAL TEMPERATURE EXCEEDS THAT AT WHICH CORROSION ATTACK OF THE METAL OF THE HIGH STEAM TEMPERATURE SUPERHEATER WOULD OCCUR IF THE FUEL USED TO GENERATE THE SECOND STREAM OF COMBUSTION GASES WERE OF A CORROSIVE CHARACTER. 